Build script unstable and inflexible

The build script falls over if any information for the STL selector is missing or if the extras repository structure changes.

This means we need to lock extras or risk all microscope pipelines failing.

I think the STL selector was nice, and was a stop-gap solution for when we had many configurations and confusing instructions. I feel that now it makes the build script incredibly inflexible and unstable and means we have to doubly define information (i.e. the STL selector could have different set of STLs from GitBuilding)

We need to fix this before release.